Engineering of the interface of a solid surface with contacting liquids to increase the liquid mobility is of great interest due to its broad technological implications ranging from biomedical to mechanical applications. Recent years have seen rapid and noticeable advances in the design and fabrication of super-repellent surfaces as well as lubricant-infused surfaces as two different approaches in solving this extremely challenging problem. Super-repellent surfaces, relying on an air layer trapped at the solid-liquid interface, while promising, suffer from inherent limitations that severely restrict their applicability. Repellency on such surfaces is prone to failure under pressure; retaining the repellency against low surface tension liquids entails the incorporation of complex (and usually fragile) topographic features with fabrication and scalability problems; surfaces are not fully transparent; and the surfaces can be fouled by contaminants. Similarly, despite their great potentials, lubricant-infused surfaces have not been considered a viable solution since their performance can be threatened over time by lubricant evaporation or its loss under flow conditions.
In a new direction, recently, nearly smooth solid surfaces grafted with covalently attached flexible hydrophobic groups have shown high slipperiness against contacting liquids. However, utilization of these surfaces in biomedical applications is highly restricted due to their vulnerability against fouling (induced by their hydrophobic nature). As well, performance of such surfaces in condensation heat transfer is debatable due to the low nucleation rate on hydrophobic surfaces. In striking contrast, hydrophilic surfaces (i.e., surfaces with contact angle θ<90°, see FIG. 1a) have attracted tremendous interest in such applications due to their superior antifouling capability and high nucleation rates. However, lack of slipperiness on conventional hydrophilic solid surfaces (see FIG. 1a, a water droplet cannot slide on a hydrophilic surface even when it is tilted by 90°) hinders the utilization of such surfaces in many applications.
While there are no prior experimental reports on hydrophilic, yet slippery solid surfaces, recent molecular dynamics simulations have demonstrated that hydrophilic surfaces with very high physical homogeneity (i.e., negligible surface roughness), very high chemical homogeneity (i.e., highly uniform distribution of hydrophilic molecules on the surface) and sufficiently low inter-tether distance D allow water molecules to smoothly slide past the surfaces. Although slippery surfaces that are hydrophobic and lubricated have been fabricated before, the materials and methods to experimentally fabricate slippery solid surfaces that are hydrophilic have never been disclosed. Therefore, there is a need for fabricated hydrophilic and slippery surfaces.